Compressor



Jan. 2l, 1947. v PAVLECKA ETAL 2,414,551

COMPRES SOR Original Filed July 21 1941 2 Sheets-Sheet l Mn S Sm v. R P mwah M nm 9 m n .www n WHO .nN\ RN. MK W .Svh www@ ww S vnd .n 3 R. www S No o Nh, QS g 0S mnv\ MM s Nm 1--- Nw ww" mmf Iii. |1.|| -if 4 on Ir \v OQ m@ 1 o. \`l. MN m@ xv@ wm Nm Jam 21 1947 v. H. PAvLEcKA Erm. 2,414,551

COMPRESSOR Original Filed July 21, 1941 2 Sheets-Sheet 2 /NvEA/roRS.

VLAD/M/R H. B4VLEC/(A. 43 JOHN K. NORTH/POP.

ATTORNEYS.

Patented Jan. 21, 1947 COMPRESSOR Vladimir H. Pavlecka, Pacific Palisades, and John K. Northrop, Los Angeles, Calif., assignors to Northrop Aircraft, Inc., Hawthorne, Calif., a corporation of California (')riginal application July 21,` 1941, Serial No. 403,338. Divided and this application October 6, 1941, Serial No. 413,781 v i 6 Claims. l

This invention relates to compressors for gas turbines of the continuous combustion type opyerating at substantially constant pressure, ac-

cording to lthe Ericsson thermodynamic cycle, sometimes erroneously called the Brayton cycle. Although the theory of such -turbines has long been known, and some have been put into actual commercial operation as stationary power plants, relatively little has been so f ar accomplished in the development of gas `turbine power plants for driving purposes in transportation, particularly for airplane propulsion.

This application is a division of our prior application Serial No. 403,338, led July 2l, 1941, entitled Gas turbine.

The present invention deals with a compressor used in a gas turbine power plant specifically conceived for airplane propulsion use, although its utility can be extended to other elds where compact, light and eflicient compressors are in demand.

A gas turbine power plant becomes commerl cially attractive when its thermal efliciency reaches values comparable lto the elliciency of existing reciprocating thermal power plants. It will be apparent to those familiar with thermodynamics that good -thermal eiciencies of gas turbines can be obtained if either the maximum cycle temperature is increased or the thermodynamic eiiciencies of the -turbomachines improved or, preferably, if both are increased at the same time.

So far, gas turbine power plants have not come into wide use partially because of diillculties with metals under stress at high temperatures of combustion gases, and also due to the insufficiently high thermodynamic efficiencies of turbines and compressors obtainable with the existing knowledge of aerodynamics.

To those familiar with the problem, it will be apparent that in the thermodynamic expression for the overall thermal efficiency of a gas turbine power plant, the thermodynamic efficiency of the turbine is a Very significant and determining factor, more so than the thermodynamic efciency of the compressor which it drives.

The attainable thermal efliciencies of gas turbine plants rise rapidly with the increase of the maximum cycle temperature at which the propulsive gases may be used in the blading, so rapidly in fact, that the difference between a maximum cycle temperature of say 1450 F. and 1200 F. may constitute the difference between a commercally advantageous design and one which is inferior in (Cl. 23o- 119) performance to other types of existing thermal prime movers.

The difllculties attendant upon high temperature of the propulsive gases manifest themselves in various ways. One of the most important oi' these is the creep or gradual deformation of material under stress. The rate of creep varies With the stress and also with the temperature. The variation with stress is approximately linear, but the variation with temperature is exponential, i. e., the rate of creep increases in geometric ratio while the temperature is increasing in arithmetic ratio. The phenomenon constitutes the greatest obstacle to the use of high temperature thermodynamic cycles in gas turbine plants. the turbine blading or its supporting structure creeps, there will eventually come a .time when the clearance between the rotor and stator disappear, and replacement of these parts is necessary. Therefore, f the turbine be designed` with small clearances and very low stage leakage for initially high thermodynamic efficiency, and the temperature of the gases in the rst stages of the turbine also be high for the same purpose, it is quite possible that the creep rate will be so high that replacement of the turbine components will be necessary in a relatively short time, and that savings due to high thermal efficiency will be more than oiset by large rebuilding costs.

Reduction of unit stresses, of course, delays this effect, but it also increases weight and size of moving parts, and hence not only increases the initial cost, but defeats the hopes for obtaining a light and compact power plant.

The second major problem introduced by high temperatures has to do with heat insulation. Heat transfer to the surroundings of the machine, as well as adverse heat flow internally within the power plant become increasingly important with high cycle temperatures. External heat flow is, of course, pure waste. The internal heat transferby conduction and radiation between Various units of a gas turbine power plant manifests itself as a deterioration of lthe thermodynamic cycle. This latter loss is particularly important in a compact, light weight power plant for aircraft purposes, because here the compressor and the turbine units have to run in the closest possible proximity to one another, besides being both near the combustion chamber with its high temperature.

As is well` known, 'an air compressor operates more eiiiciently along an isothermal compression line, i. e., with internal cooling or at least with interstage cooling.` In a continuous combustion gas turbine of a compact design, constructed integrally with a compressor and a combustion -chamber and without specific means for heat in- Fig. i constitutes a one-half longitudinal sectional view of a gas turbine power plant embodying alloys which possess very low creep rates.

An important disadvantage of these alloys is their low internal cohesive friction at high temperatures, physically defined as low damping coeflicient. Parts of turbines made of these materials, e. g., bladings, are easily excited into violet vibrations even outside the region of resonance with the exciting forces, and develop fractures. The turbine structure using these alloys has to be designed with this condition in mind, viz.,-to re.. strain the blades against dangerous vibrations without detrimentally affecting the performance of the turbine as an aerodynamic machine.

Itis obvious however, that no turbine structure can be efficient without the use, in combination therewith, of4 a compressor of high efficiency so designed as to deliver to the turbine air at ing our invention, divided into three sections to reduce, length.

Fig. 2 1s a naif end view of the turbine of Fig. 1.

Fig. 3 is a diagrammatic perspective view of the blade arrangement in the path of the heated gases while changing from inwardly radial Yto out- Y wardly axial flow.

The turbine whose design we have chosen to describe as illustrating our` invention, comprises a stator shell which although sectional is indicated by the general reference character i, split axially in a horizontal plane, with the two stator halves bolted together by flanges, and carrying at one end a main journal bearing 3, of the rothe proper temperature and in the proper condition for use by the turbine. compressor is driven by the power-plant when it is used with our gas turbine, the power consumed in the compressor is of course a direct subtraction from the available power of the machine. It is therefore imperative that every precaution be taken to promote eiiiciency in the compressor and to provide the output air required, with a minimum of power absorption.

' Broadly, our present invention comprises a compressor which may be said vto have three stages. First, a turbine type of compressor in which the flow is axial; second, a rst centrifugal stage fed by the turbine typecompressor; third, a second centrifugal stage, the output of the second centrifugal stage being utilized, for example, in the turbine to be described herein.

Among the objects of the invention are: To provide a rotary compressor combining turbine and centrifugal compressor stages; to provide a novel means and method of transferring air between two centrifugal compressor stages; to provide a means and method of reducing entrance shock in a centrifugal compressor; to provide a means and method of coupling a turbine stage Inasmuch as thev compressor with a centrifugal type compressor,

and of coupling two centrifugal stages in a centrifugal compressor; to provide a combined turbine and centrifugal type compressor occupying a minimum of space and of minimum weight; and to provide a simple and eilicient compressor, ideally adapted for use in conjunction with gas turbines.

Our invention possesses numerous other objects and features-= of advantage, some of which, 'together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing our novel methods. It is, therefore, to be understood that our method is applicable to other apparatus, and that we do not limit ourselves in any way to the apparatus of present application, as we may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

/ blades.

tor, the other bearing of the rotor being carried bine power plant, this bearing not being shown.

The rotor of the power plant is a composite structure built up from a number of elements. Considering these elements in order, starting fromthe intake portion of the compressor at the right of Fig. 1, they comprise a steel hollow shaft 5 provided with a shoulder 6 bearing against a first centrifugal section 1, which in turn interlocks with a second centrifugal section 9, the shaft and the two sections being held together by bolts ll entering turbine rotor frame portion i2. The shaft portion of the rotor carries axial compressor reaction blading I0 and tapers from the.

maximum diameter at the intake end to a minimum diameter adjacent the first centrifugal compressor section. Reaction blading I 0 is spaced and stator blades I4 are attached to the compressor stator 4 between each series of rotor The axial compressor blading is of the reaction type, with profiles, generated according to the laminar flow principles, similar to Joukowsky airfoil type (reference-page 67 of Aerodynamics of the Airplane, by Clark B. Millikan). The insertion and sealing of these blades is substantially the same as that of the turbine blades and will be described morefully in conjunction with the action of the turbine. Like all other elements of the rotor hereinafter described the compressor rotor is machined inside and outside for accurate balance.

Centrifugal section 1 is prolonged laterally and carries a first centrifugal impeller I6, and the second centrifugal section 9 carries a second centrifugal impeller i 1, both formed integrally with the sections. Both of the impellers carry impeller bladings I8 and I9 respectively, which are `substantially conventional inform and preferably vassembled in the impellers by copper brazing in a hydrogen atmosphere. Each of the impeller sections carries a shoulder 20 and 2l respectively, extending parallel to the axis of the machine and these shoulders are provided with labyrinth seals 22 and 23 respectively. Seal 23 is made between the compressor housing 4 and the impeller shoulder, the seal 22 being made between the impeller shoulder 20 and the inlet duct 24 leading to the second impeller I1.

lThe periphery of impeller I6 discharges into a transfer casting 29 having a transfer passage 30 which leads into inlet passage 24 of the second impeller. The transfer casting 29 is bolted to the' compressor stator 4 on one side by bolts 3| and to the turbine shell I on the other side by bolts 32. Transfer casting 29 also carries a first impeller enclosure 34, and a second impeller enclosure 35, both adjacent and surrounding the impellers. The right-hand portion of enclosure il aids in supporting seal 22 for the second impeller. At the lower portion of inlet passage 24, casting 29 is sealed to centrifugal section 1 by a labyrinth seal 99.

A specialentry blade 31 is positioned to receive the air from compressor blades I and I 4, and to direct the air into the rotating blades of the nrst impeller I8, with a radial component of flow to eliminate entry shock. Thus the angle oi' the entry blade 31 adjacent the impeller blades I9 is at an acute angle to the shaft looking from the intake end. Similarly a stationary blade 39 is attached to transfer casting 29 to direct the air l into the second impeller I1 at substantially the same angle to likewise eliminate entry shock at this point.- v

Immediately around the periphery of the second impeller I1 is positioned a compressor outlet passage 39 curved to direct the air in a direction parallel to the axis of themachine into the turbine portion, and into an annular and outer expansion passage 40. All of the rotative elements so far described operate at relatively low tems peratures and can be machined from austenitic type of steel.

The next section I2 of the rotor, however, carries the linitial turbine stages and is turning in gases of high temperature. For this reason this section is preferably constructed of a low creep rate alloy, such as that designated in the trade tion I2 is made of this alloy and as previously explained, is held to impeller section 9 by bolt Il and is positioned by means of mating surfaces 4I. The turbine rotor is made in sections, the iirst section I2 being providedwith a web 49, and terminating in a groove 44, which, with opposing groove 45 of the final' section 4 8 of the turbine rotor forms a channel for the reception of an annular retaining shoulder `46 forming a part of a power stub shaft 41 from which the power is taken. The rotating portion of bearing 9 is mounted on shaft 41, matching the stationary portions ofthe same bearing attached to the turbine case I. The two rotor sections I2 and .46 are held on annular shoulder 46 by a bolt 59. Turbine impulse blades I are attached to the rotor sections between rows of reaction blades 52 attached to the stator. Both rotor and stator have special entrance blades which will be later described.

It can be seen that the rotor thus described, although it contains no continuous central shaft, can be built with great accuracy and rigidity. The mating surfaces of the adjacent sections of both compressor and turbine rotors can be easily nished to a high degree of precision. It is these engaging cylindrical surfaces which determine the alinement of the entire rotor.

Turning our attention next to the blading. One of the major energy losses in turbomachines is the external and internal leakage. The external leakage loss is practically eliminated in our invention, because the high pressure end of the compressor is directly adjacent to the high pressure turbine end, without the presence of an interveningbearing as is the case in other gas turbines. We not only do not have to resort to extremely complicated and delicate seals to retain high pressures of gases at high temperatures, but weractually create conditions under which we are able to meter a certain amount of air leakage for cooling and insulating purposes, as will be apparent later on in this description. l

Internal leakage is not combatted in the Prior '30 as K-42-B alloy. Thus, the rotor frame porart of turbine designing at all, in spite of the fact that it adversely innuences the thermody--V namic emciency of the turbines." Internal leakage can be defined as leakage of gases past the o blade stages without doing useful work o'n the blades. It not only constitutes a direct and nonrecoverable loss of gas energy, but ,also has an adverseindirect effect upon the flow oi gases through blade cascades, 1n that it.' disturbs the l0 two-dimensional flow of gases an'dfilnduces it to deviate outwardlynin the directior ofthe blade tips. This flow then sets up a tipvortex at the end of each blade which is the source of the socalled induced drag. For bladesfiof low aspect l5 ratio, such as are common in the" high pressure stages of elastic fluid turbines, this'drag is of considerable importance, because in elastic iiuid tur- -bines the absolute velocity of the leaking gases is higher than the relative velocity ot the gases expanding in the blade cascades and the tipA vortex is then extremely strong. t

In our invention the individual blade stages are sealed along their peripheries by labyrinth ilanges to reduce the stage' leakage to the very minimum, and also to remove the place of leakage away from the tips oi' the blades, so that leven if Ia slight leakage does occur through the labyrinth seal, it has only a remote influence on the 'two-dimensional iiow through the blade cascades. 'I'he ends of both stationary, as well as rotary blades, are encastrd by a Uhroud 69 and 6I re spectively. Each of these ripheral shrouds carries a plurality of stage sealing flanges -92,bearing on seal seats 99 on the opposing member. The sealing flanges are originally machined 'to a close .flt and., are run vin by/external power before the turbineisactually used for the rst time. We apply'this methodoi' stage sealing to the turbine stages as well as to the axial ow compressor 40 stages, andseals 22, 23 and 36 previously described -are of this type.

The pressure developed at the outlet-of the nal centrifugal stage is somewhere in the neighborhood of 7 atmospheres, and the compression, being as nearly as possible adiabatic in a power plant for transportation purposes, the exit air temperature runs in the neighborhood of' 500-600 F. At such temperatures organic gasketingl ma terials are 'not ordinarily satisfactory, in view of the accurate ts and small clearance to which the power plant is preferably constructed. Leakage at the'various stator housing joints is, in our' invention, prevented by special self-sealing rings 92, as described inthe patent' to Daniel F. Egger, No. 2,303,114, of November 24, 1942. These sealing rings are made of elastic material such as spring steel, and are convex in the direction along the joint from which the fluid pressure is applied. The internal pressure tends to atten out the sealing ring and forces it to make a better contact with the housing so that the tightness of the seal improves with increasing pressure.

The maior diameter wall 1I of the annular duct 4I) terminates in a steel ring 99 to which the end section 10 of the housing I is bolted. 'I'he inner wall 1I of` the duct is welded to this ring. Also welded to the ring 69 immediately outside of the wall 1I, is an emular fitting 12 which carries a series of spaced polished stainless steel reflector 70 plates 13 extending between wall lI and housing I. The reflector plates, although generally circular, are not accurate circles, but are preferably waves in outline. These reflector units are preferably made in segments to permit pressure equalization between the plates. This pressure is adback toward thecenter of`the device, and there is a rather high' temperature gradient between the inner and outer walls 'of the duct 40. This minimizes heat loss and tends t'o keep the outer casing of the turbine relatively cool. The me-y chanical strength to withstand the pressure within the duct is provided by the outer wall, and

since this is relatively cool compared with the inner wall it can be much lighter than it would be were the stress carried by the hottr member.

The terms inner and outer walls, as used thus far, have referred to the multiple elements of the major diameter wall `o'f the'annular duct 40. The

'minor diameter wall of this duct is formed by a similar and slightly thicker multiple structure designated generally'by the reference character 80. This minor diameter wall also forms the outer or major diameter wall of areiiexed inner duct which forms the combustion or burner chamber 8|. The pressure on the two sides of the wall 80 is nearly the same, and the mechanical strength of this wall need not be great. Here again polished steel plates 13 are positioned in the wall to reect heat tending to radiate outwardly.

The duct 8l reexes into the inner combustion chamber 8| through the semi-toroidal end casting 10 which is outwardly bolted through ring 84 to the ring 88. The center of the toroid is mounted on the end of the stator core 88 supporting the remainder of the toroid by means of radial, spaced spoke-like vanes 88. The core 88 projected toward the compressor serves as support for ladividing wall 81 between the combustion chamber and Ythe core. Casting 10 is outwardly covered by end portion 82 of housing I.

Gases owing from the outer duct 40 and combustion chamber 8| are guided through end casting 82 by aseries of spaced and oiset vanes 88,

-preferably o! airfoil section, supported on the radial vanes 88. Vanes`88 also lserve as an end support for dividing wall 80., The vanes 88 are also preferably of airfoil section, and carry burner jets 88 which discharge at the trailing edges ef the vanes 88 in a conical spray whose general direction isparalleltc the airflow. At that portion, as is described in the patent to Dallenbach .and Northrop, No. 2,296,023 of September 15, 1942, there are preferably at least six of these burner jets disposed equi-angularly about the periphery of the turbine axis, and each jet discharges into the space between two divergent vanes 80, o! airfoil section, and in stalled position with relation to the airflow.

The conical spray from the burner Jet impinges against igniter elements 8| set into the surface of the vanes 80, and is ignited thereby. There is an intense turbulence set up by the action of the stalled vanes exactly as such a turtremely high, on the order of 3,000 F. to 3,600 F..

A portion of theair, however, passes between adjacent pairs lofvanes 88, and does ynot take part '8 in the combustion. After passing thetrailing 82 forming a part ofcasting 28. angularly Joined to-the minor diameter wall .80. This end wall carries a series of inwardly projecting spokes 88 of airfoil section. These spokes carry guide vanes 48,8 for directing the axially owing gases radially inward. 'Ihe inner ends of the spokes 88 are attached to an abutment ring 81 which forms an abutment for the inner wall 81 of the combustion chamber, and also provides a nozzle support ring 88 which forms one wall of the'radial duct 88 into which the gases from the combustion chamber discharges. The opposite wall is formed by an extension |00 of end wall 82, this extension Joining casting 38 adjacent impeller I1 to provide a thick hollow diaphragm |0|. phragm I0| is internally provided with a plurality of polished insulating plates |02 similar to plates 13. Radial 'expansion nozzles |08 are mounted between abutment ring 88 and extension |00. The abutment ring 88 is also secured to the main reaction turbine frame core 88, which comprises a plurality of annular integral diaphragms |01, which areintegrally connected by and support a seriesv of accurately cylindrical blade seats |08, which -form anv expanding,

Astepped internal cone. Blade support rings |,|0,

provided with spring anges I, are press-tted within the blade support seats, as is described in a copending application of Pavlecka, Serial No. 385,105, flied March 25, 1941. Each of the blade support rings carries the previously mentioned row of reaction type stator blades 82, and the' labyrinth seal seats 83. As in the case of the the blades, they are kept in compression by an annularly curved compression spring ||8. This spring bears against ring portion |20 of housing to which both the toroidal casting 82 and the core 88 are bolted. 'I'he ring |20 is supported by vane-like spokes |2| which connect the ring |20 to an inner casting ring |22. Casting rings |20 and |22 dene a ilaring annular exhaust passage for` the spent gases. 'Ihe casting |22 is protected between spokes |2| from direct action and erosion by these gases by a deector plate |24. The stationary part of main bearing 3 is mounted on the casting |22 and beneath the deflector plate |24 is an air passage I through which cool air is drawn from around the exhaust to cool the bearing 3,-this air passing through ducts |25 and |26 to enter the exhaust stream. Oil is supplied to bearing 3 through oil duct |23.

It will be obvious from what has been said at the outset of this specification that the critical point in this turbine. as in all past turbines, is the initial stage where the temperatures are highest. The propulsive gases are expanded very Diarapidly in the stator nozzles |06. As will be seen in Fig. 1, these stator nozzles are short, they are supported at both ends, they are not subjected to rotational stresses and as will be seen later, are air cooled. They are made of anticreep alloy. It will be recognized that their strength is ample lor the duty to which they are subjected. Owing to the large expansion that takes place in these stator nozzles I |16, the temperature of the propulsive gases drops rapidly therein, and

by the time these gases have reached the iirst rotor blades their temperature may be dropped from 150 to 200 F., depending upon the detail of the design. However, in spite of the cooling which has taken place in the expansion through the stator nozzles |06, the gases impinging upon the iirst rotor blades |21 are very hot, and these blades are, of course, subject to rotational stress. They are radial impulse blades and their unconventional design is to enable themto withstand these stresses.

This design is predicated upon the fact that unlike most rotor blades, they are supported at both ends. The inner end, closest to the com pressor is supported on a ring |29 forming a part of first rotor section |2.

The other, or outer endsof the rotor blades |21 are supported by a ring |3| which connects with a second ring |32. The latter ring forms the shrouding for a second set of rotor blades |34 set at right angles to rotor blades |21. The rings |3| and |32 are tied together by a third ring |35 which forms the seat of labyrinth seal |35 carried by the stator structure.

The change in direction between rotor blades |21 and rotor blades |34 is aided by curved airfoil blades |31 mounted on rotating standards |38, the latter being connected to diversion wall |39 connecting the bases of rotor nozzles `|34 with ring |29.

The rotor thus carries two successive sets of bladings, the radial blades |21 and the axial blades |34, with no stator bladings intervening between the two. The lrst set of bladings |21 is adapted for inwardly radial flow and is impulse type blading. The second cascade of blades |34 may be of either of two types. In one case these blades may be purely directional vanes which give a peripheral direction to the axial stream of gases coming from the radial blades f |21. In this case a high exit velocity from blades |21 would be used, and in many cases such a high velocity would be undesirable because of high frictional and turbulence losses in the duct between the two blade cascades. We prefer. therefore to use a lower exit velocity and to design blades |34 as expansion nozzles, and thus add to thegas velocity of the gases entering them. If this latter construction is used, the gases from blades 34 will discharge into the rst row of characteristic reaction type blades |31 mounted on the stator.

The unconventionality mentioned above in this preferred structure is three-fold. First, the flow is inward instead of outward as in the ordinary radial ow turbine, This permits the use of an impz'nging angle against the rst rotor blades |21 of only 91/2 instead of the more usual 14, which results in a very high peripheral efficiency of the stage. Could cantilever blades be used, so that the blades 21 could be of conventional form, it would be theoretically possible because of this low angle to get a thermodynamic efficiency of approximately 84 out of this stage. In steam turbines, where the temperature is lower, such an inward flow axial impulse stage would be highly desirable. This fact is not self-evident, and where radial iiow has been used in turbines in the past, it has been, so far as we are aware, always outward. With outward flow the impinging angle is increased, above that possible with axial How turbines, but since outward ilow gives room for the natural expansion of the gas, outward ilow is the expedient to which the en gineer naturally turns.

Here, however, instead nf the gases discharging from the buckets |21 into stator blading, they discharge into a passage and are guided from the radial into the axial direction by the walls of this passage plus the guide-vanes |31, rather than having the buckets |21 arranged so as to themselves guide the exhaust gases in such direction. The angles of admission of both series, are substantially the same, relative to the blades and the blades receive the gases almost tangentially and discharge them almostradially. The exhaust angle is preferably such that Athe discharged gases are vrotating with the rotor structure at an angular velocity which is equal to the mean velocity at the entrance of the expansion nozzles 34 on the rotor, i. e., the absolute velocity of the gases is slightly less than that 0f the buckets |21.

The effect of the relationship set forth is a great apparent reduction in the efficiency of this stage of the turbine, but this reduction in efficiency is more apparent than real, for the discharged gases still retain a velocity which is recovered as mechanical work in the succeeding stages. The calculated eiiiciency of the radial stage shown is only 58.6%, but this does not mean that 41.4% of the energy of the adiabetic expansion of the gases is lost, but only that 25.9% of the useful mechanical energy of the gases is not intercepted in this stage but passes on. to the` axial reaction stages to do useful work.

As has been stated, the temperature drop in the radial stage is approximately 150 to 200 F.

'The expansion which takes place in the nozzles |34 further reduces the temperature of the gases, so that a relatively conventional design is used in the stages from this point on to the exhaust.

Even with the blading |06 and I 21 supported at both ends in order to provide long life, it is desirable that these bla-des should not reach the full temperature of the gases. and means are therefore provided for cooling them. .A space |40 is provided around diaphragm |02 which admits a predetermined quantity of air directly from the exhaust of the nal compressor sta ge to the compressor side of ring |29 supportingr the blading |21. The blading |21 is provided with longitudinal holes |4|, and the air admitted Athrough the space |40 passes through these holes, thus cooling the blades |21, this air discharging between blades |34 and |31', Space |40 also continues above blade |21 and a small amount of air may enter the space between blades |06 and |21.

In addition an air port |50 is provided at the end of duct 40 to sap off some of the air before it has been heated so that it may enter the space between plates 13 in the minor diameter wall 80, pass along the plates to a space I5I between end wall 92 and plates |02 of the main diaphragm |0| between the compressor stage and the turbine stage, this air discharging through holes |06' in each nozzle |06 into space between wall 81 and core from which it is vented through part 91 t0 the adiacentedges of blades |06 and |21. Air

l1 also enters between wall 81 and core 85 adjacent the burners and is vented through part 81,

Furthermore, space |40 is connected with the space |52 back of deflector plate |39. this air cooling theplate and exhausting in back of rotor blades |34. Thus, blades |06 and |21 are cooled by air passing through the metal thereof. The leading edge of blade |21 is cooled by air fed thereto and the leading edge of blades |31' is cooled by the vented air a-s it passes into the reaction stages. compressor directly, the stator air is sapped oi adjacent the burner.

In discussing the action of the turbine in its various important aspects, it will be seen first that the compressor end of the machine is a combination of a turbine type compressor and two centrifugal stages. Important in this regard is the transfer of the air from the preceding stages into the impellers of the centrifugal stages. in that the transfer takes place with a radial oomponent of Alow and therefore entrance shock is reduced. The reflexed vduct 30 between the two impeller stages is expanded as little as possible, with no expansion whatever during the turning of the gases from outwardly radial flow to inwardly radial flow. Even then, during the remainder of the inwardly radial inlet passage 24 to the second impeller stage. very little enlargement of the duct occurs. This type of connection between the t'wo impellers transfers the air between the two impellers with substantially no expansion, and therefore little change from dynamic energy into potential energy occurs. The air, when it reaches the input to the second impeller stage retains substantially al1 of its dynamic energy produced by the first impeller stage. Similarly, the outwardly moving radial air, coming from the second impeller stage, is turned to axial direction without substantial loss of dynamic energy. No great change to potential energy takes place until the air reaches the annular duct 40. At this point expansion will take place and the air will reach the burners in proper condition for mixture with the fuel. Furthermore, the compressor and turbine stages are insulated against the hot gases by the use of the relatively thick diaphragm IUI formed by polished plates |02, so that there is relatively little heat interchanged between the combustion chamber 8| and the compressor.

In addition, both the major and minor diameter Walls of the outer passage 48 are insulated by polished plates 13.. 'I'he combustion chamber 8| is further insulated from the outside of the turbine because it is surrounded by passage 40 in which the air itself acts an insulating cloak to reduce the escape of heat. The gases are heated in axial flow and are then turned iny wardly for a first and radial contact with the rotor `structure and then are turned axially, without the interposition of stator blades, to do additional work on rotor blades |34. The gases thereafter in axial flow act in conventional manner on the bladings 52' and 5|, with the exhaust passage continually becoming larger in crosssection up to the exhaust. Bearing 3 is not only cooled by air but is also protected from the heated gases by the structure used. Creep throughout the machine is prevented from causing any significant losses first, by using the proper metals for the bladlngs and second, by the use of shrouds, which provide `under surface air ducts for insulation of main structural components from high temperature gases.

Therotor air comes from the Thus all the advantages heretofore pointed out` are attained in a relatively light structure suitable, for example, for being used as a compact prime mover for an airplane or in any other place where lightness and efficiency are desired.

The S-shaped gas path in the turbine greatly reduces overall length, and the inward ow per`- mits a structure in which radiation losses are greatly reduced, and in which the compressor discharge air separates the combustion chamber from the outside air, still further reducing heat loss. The use of airfoil bladlngs increases eiliciency wherever used, and the structure as assembled leads to a system of internal cooling where needed without heat loss. I

We claim:

1. A compressor having a rotor and a surrounding stator, a group of rotor and stator reaction type turbine blade rings,`shaped to pass air axially along said rotor and stator, a centrifugal impeller mounted on said rotor and having substantially radially extending impeller blades, said stator forming an air passage with a portion of said rotor intermediate said group of turbine blade rings and said impeller, said passage at the entrance` to said impeller being shaped to impart to said air from said turbine blades a radial component of flow into the spaces between said impeller blades to reduce entrance shock.

2. A compressor having a rotor and a surrounding stator, a group of rotor and stator reaction type turbine blade rings, shaped to pass air axially along said rotor and stator, a centrifugal impeller mount-ed on said rotor and having radially extending impeller blades, said stator forming an air passage with a portion of said rotor intermediate said groupof turbine blade rings and said impeller, said passage at the entrance to said impeller being shaped to impart to said air from said turbine blades a radial component of iiow`into the spaces between said impeller blades to reduce entrance shock, the leading edges of said impeller blades being positioned substantially at right angles to said flow.

3. A compressor having a rotor and a surrounding stator, a group of rotor and stator reaction type turbine blade rings, shaped to pass air axially along said rotor and stator from said inlet, a centrifugal impeller mounted on said rotor and having substantially radially extending impeller blades, said stator forming an air passage with a portion of said rotor intermediate said group of turbine blade rings and said impeller, said passage being shaped to at least partially turn said W from an axial direction to radial direction before striking the leading edges of said impeller blades to reduce entrance shock thereon.

4. In a compressor supplied with an axially moving airflow, a 'centrifugal impeller having radially extending blades, a shroud for said im,- peller, means on said shroud forming, with a basal portion of said impeller, a conduit directlng air into said impeller blades, and an entry blade disposed in said conduit adjacent the impeller blades at an acute angle to the compressor shaft as viewed in the direction of said axial flow.

5. Apparatus in accordance with claim 1 wherein a second and similar impeller is provided on said rotor, said stator being shaped to form an annular conduit receiving air peripherally Afrom the first impeller, and shaped to direct the peripheral air first axially, then inwardly radially and finally axially to deliver said air to the bases of the rotor blades of the second im- 13 peller, said conduit being shaped adjacent the bases of the rotor blades to deliver air thereto with a. radial component of flow.

6. Apparatus inv accordance with claim 1V wherein a second and similar iimpeller is provided on said rotor, said stator being shaped to form an annular conduit receiving air peripherally from the first impeller, and shaped to deliver the peripheral air rst axially, then inwardly 

